The invention relates to a process for the preparation of thermoplastic polyurethanes (TPU) with a low tendency towards granule surface tackiness.
It is known that granules of aliphatic TPU based on isophorone-diisocyanate (IPDI) or based on 4,4′-, 2,4′- and 2,2′-dicyclohexylmethane-diisocyanate and isomer mixtures thereof (H12-MDI) tend towards surface tackiness (See, e.g., EP-A 1090940), even if these TPU contain the known waxes and/or release agents.
This surface tackiness, which also arises in soft (Shore A hardness<80) TPU based on hexamethylene-diisocyanate (HDI) and methylene-diisocyanate (MDI) also occurs in TPU based on H12-MDI and IPDI in hard products in the region of a Shore D hardness of >40.
Shaped articles of these aliphatic TPU are employed in all instances where a good light stability and high transparency are required, such as flexible windscreens (e.g. cabriolet rear screens), an elastic intermediate layer in safety glazing, etc.
Soft thermoplastics, such as aliphatic TPU, must be stored and transported in drums at temperatures of approx. 10-40° C., since after storage and transportation the granules must be still be free-flowing (no agglomeration) so that they can be handled during further processing. Furthermore, aliphatic TPU must be dried before processing at temperatures of >50° C. in order to avoid a molecular weight degradation during the processing, and agglomeration after the drying should also be avoided (maintaining of the free-flowing properties).
Simple application of known waxes and/or release agents to the granules is not sufficient to obtain free-flowing granules which are also still free-flowing after any drying.